1. Field of the Invention
The invention relates to a sheet feeding method for image formation apparatuses, such as printers, facsimile, copiers, having a plurality of cassettes with a transport path for the sheets stored in an upper cassette also serving as a portion of a transport path for the sheets stored in a lower cassette.
2. Description of the Background Art
A conventional image formation apparatus has a sheet transport path for an upper sheet cassette also serving as a sheet transport path for a lower sheet cassette. In the image formation apparatus, the lower cassette initially feeds sheets, and when the sheet of the lower cassette run out, the upper cassette then feeds sheets. The lower cassette resumes feeding sheets when the lower cassette is resupplied with sheets with the upper cassette feeding sheets. Thus sheets are fed successively.
However, the image formation apparatus has a disadvantage that insertion or exit of the upper cassette into or from the body of the apparatus while the lower cassette is feeding sheets results in jam of the sheets being transported from the lower cassette and thus lowers image formation efficiency. In order to overcome the disadvantage, the sheet feed device of the image formation apparatus disclosed in U.S. Pat. No. 5,155,537 prohibits insertion and exit of the upper cassette while the lower sheet feeding cassette is feeding sheets. Otherwise the sheet feed device is provided with signaling means for signaling to the user whether insertion and exit of the upper cassette is allowed.
For the sheet feed device described above, however, the upper cassette cannot be resupplied with sheets while the lower cassette is feeding sheets. Thus, successive image forming operations cannot be effected when sheet feeding is switched from the lower cassette emptied to the upper cassette that does not contain any sheets.
Furthermore, if the user does not notice that the sheets of the lower cassette run out and fails to resupply the lower cassette with sheets while the upper cassette has sheets and is feeding them, the current image forming operation is terminated when the sheets of the upper cassette run out.